Time-of-flight (TOF) mass spectrometers derive calculations of ion masses from direct measurements of ion flight time within a flight tube, where the mass is proportional to the square of the flight time. The flight time is directly proportional to the length of the flight tube, and any minute change in the length of the flight tube due to temperature fluctuations correspondingly changes the measured ion flight time and results in inaccuracies in the calculation of ion mass. For example, a 1 μm change in a one-meter-long flight tube translates into a 2 ppm (parts per million) change in calculated mass.
There are two conventional techniques for dealing with this problem. The first is to attempt to stabilize the temperature of the flight tube by isolating it and insulating it from ambient temperature fluctuations. One significant problem with this method, apart from the inconvenience of physically isolating and insulating the flight tube, is that the ion beam traveling within the flight tube can itself cause increase in the temperature of the walls (usually composed of stainless steel) of the flight tube, so that no amount of structural isolation and insulation can completely eliminate temperature fluctuations. The second conventional technique is to compensate for any variation in the length of the flight tube by using a reference mass to calibrate analyte ion mass calculations. A shift in the calculated mass of the reference compound can be identified and this shift can be accounted for in the calculations of the analyte ion masses. However, with this technique, the inclusion of the reference mass can interfere with the detection and analysis of analyte ions because the mass of the reference compound may overlap closely with the mass of a detected analyte ion or one of its isotopes. This may result in an erroneous analysis of the composition of analyte compounds.